Wireless passive sensors do not require any power source for operation other than the interrogation signal from the reader. The advantages of passive sensors over active and semi-passive sensors are that they are potentially more inexpensive and their operation conditions or life-time are not limited by the power source, such as a battery or energy harvester.
Passive wireless sensors can be divided into digital and analog sensors. The IC (Integrated Circuit)-based RFID (Radio Frequency Identification) utilizes digital logic which offers highly sophisticated features such as anti-collision protocols and non-volatile memory. The IC RFID is mostly used for identification, but can also be equipped with a sensor element.
As compared to digital architecture, analog sensors may potentially offer better energy efficiency. This is because the digital electronics uses a fraction of the received energy to operate the IC, whereas analog sensors can theoretically backscatter all the received energy. In addition, the read-out distance of digital sensors is often power-limited while that of analog sensors is limited by the signal-to-noise-ratio. Therefore, the read-out distance of analog sensors can be increased by increasing the integration time. Due to these reasons, analog sensors have advantages in certain special applications.
Analog sensors include surface acoustic wave (SAW) based RFID, resonance sensors, and harmonic sensors. SAW sensors utilize an interdigital transducer patterned on a piezoelectric substrate to convert the electromagnetic energy into a SAW. SAW is then manipulated with acoustical reflectors, transformed back to electromagnetic energy, and radiated to the reader device.
The measured quantity affects the propagation properties of SAW on the piezoelectric substrate. The need to use a piezoelectric material for a sensing element limits possible applications. In addition, the smallest line-width of the IDT structure limits the highest operation frequency of a SAW tag to a few GHz.
Resonance sensors consist of a simple resonance circuit, whose resonance is sensitive to a measured quantity. These sensors require a near-field coupling to the reader, which limits their read-out distance to a few centimeters. Another hindrance is that their resonance may be affected by a proximity to conductive or dielectric objects. Resonance sensors are used for example to monitor moisture in building structures, strain, and blood pressure.
Harmonic sensors backscatter the sensor data at an harmonic frequency of the interrogation signal frequency. The concept was first proposed for telemetry [9]. Later, harmonic sensors that double the interrogation frequency have been used to track insects in biological and agricultural studies and to locate avalanche victims. Recently, an intermodulation communication principle is proposed for sensing applications. In this principle, the sensor is actuated by two closely located frequencies and the sensor data is backscattered at an intermodulation frequency. As compared to harmonic principle, the intermodulation communication offers smaller frequency offset. Small frequency offset facilitates circuit design and compliance with frequency regulations.
There are some drawbacks with the previously published sensors utilizing the intermodulation communication principle. First, the sensor presented in uses a MEMS element simultaneously for mixing and sensing, which implies that the MEMS needs to be compromised between the two functions. Similarly, the sensor presented in utilizes a ferroelectric varactor both for mixing and sensing. In addition, the resonance of the ferroelectric sensor may be affected by a proximity to conductive or dielectric objects.
The sensor platform presented in incorporates a separate mixer and it can be equipped with a generic capacitive sensor element. A hindrance of this design, however, is that an inductor is used to obtain a resonance at a low frequency. A self-resonance frequency of the inductor limits the smallest achievable frequency offset and the quality factor of the inductor limits the conversion efficiency and thus the read-out distance. Due to these reasons, the frequency offset needs to be relatively large and the read-out distance is limited. Large frequency offset impedes the compliance with frequency regulations.